1. Field of the Invention
This invention relates to a power supply for an electric vehicle, and in particular to a power supply which can be made compact and lightweight.
2. Description of the Related Art
Various types of electric vehicle are known which run using power stored in an electrical storage device to drive a motor.
Examples of such vehicles are an internal combustion engine-electrical storage device hybrid electric vehicle wherein a motor linked to the main shaft of the engine and an electrical storage device are connected via an inverter circuit and power is transmitted between them, a generator battery-electrical storage device hybrid electric vehicle wherein power is transmitted between a fuel battery or solar cell and an electrical storage device via a converter circuit, and an electric vehicle which is driven only by an electrical storage device. These electric vehicles are provided with a so-called regeneration circuit which converts the energy of braking into electrical energy and charges the storage device.
Typical storage devices used in these conventional electric vehicles are batteries such as lead accumulators or nickel hydrogen cells and high capacity condensers such as double layer capacitors.
This storage device must be able to provide sufficient energy to improve power characteristics and steering to match the acceleration performance of the vehicle, and improve acceleration holding distance. It must also have high power charging characteristics in braking regeneration.
However in the above conventional electric vehicle, only one electrical storage device is used, so if it was attempted to satisfy all the above criteria, the storage device became bigger and heavier, and it was very difficult to mount it on the vehicle. On the other hand, when the storage device was made small so as to be able to mount it on the vehicle, it charged and discharged too rapidly or overcharged and overdischarged when it output power or was charged with regenerative power, and this led to a shortening of the lifetime of the device.
In particular, in the case of a hybrid electric vehicle where an internal combustion engine or generator type battery are mounted together with the electrical storage device, there are more limitations on space and weight, and hence there is an even greater need for compactness and lightweightness.
As a solution, Japanese Patent Laid-Open Publication No. Hei 5-30608 discloses a high capacity condenser used in conjunction with the electrical storage device. In this conventional example, charging and discharging during large accelerations and decelerations of the vehicle are mainly handled by the high capacity condenser.
However, the energy density of existing high capacity condensers is only 1/4-1/35 of that of a conventional storage battery, so the energy density of the whole system decreased as a result. The condenser has a high output power per unit weight, i.e., output power density (W/kg), and a high regenerative power charging capacity, i.e., regenerative power density (W/kg). However the specific gravity is only approx. 3/5 of a conventional storage battery, so the output power density (W/L) and regenerative power density (W/L) per unit volume were small, and the condenser became bulky.